An important geophysical issue is determining the resistivity of rocks under various conditions. To characterise the internal resistivity structure of a rock needs electrical data from many small electrodes attached around its surface. Effective measurement must overcome the high resistance of the rock itself and the high contact resistance on the rock's surface during measurement. Therefore, we developed a new method for stable, multi-point, electrical measurement on rock samples that is

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... ive at high contact and sample resistance. The method employed conductive, adhesive epoxy electrodes, which strongly attached even to dry rock surfaces and provided high conductivity. Sustained current injection for long periods into high-resistance rocks was fulfilled using a constant direct current source with high internal resistance. Accurate voltage measurement across the high-resistance rock was accomplished by differential measurement using two high input resistance voltmeters. Measurements of high resistance also require a stable measurement environment: the temperature and humidity in the laboratory were controlled using an air conditioner, a humidifier, a dehumidifier, and a vinyl tent. Signal noise arising from human activities was eliminated by the remote operation of the measuring equipment and switching terminal. We assessed the precision and stability of our new method when applied to a dry granite sample at various levels of absolute humidity. The method recorded highly reproducible measurements under all absolute humidity conditions, thus indicating its validity. The observed changes of measured values with absolute humidity showed that atmospheric moisture greatly influences a sample's resistance and contact resistance, and indicate the importance of stabilising the temperature and humidity conditions in the laboratory when taking electrical measurements of dry rocks at room temperature and pressure. Additional electrical measurement of dry granite using a simple electrode array constituted the first step toward electrical tomography measurements. A 40-electrode array acquired the potential distribution on the granite's surface in response to injected current. Sample resistivity, which was sufficient to explain the measured potential distribution, was estimated by forward modelling. This demonstrates the robustness of our method, and indicates that it is potentially applicable to electrical tomography.